Gneiss
Ancient crust taken deep enough to nearly melt, then banded by pressure into light and dark stripes.
How it forms
Rock is buried twenty kilometres or more in the root of a mountain range, at several hundred degrees. Minerals become slightly mobile and migrate into bands of like with like, a process called differentiation. The banding is the record of that migration, often folded into loops where the rock flowed while it was doing it.
Metamorphic — changed by heat and pressure
Rock taken deep, squeezed and heated until its minerals regrew in new forms, without ever melting. The new minerals line up under the pressure, which is why so many of these split in sheets or come out in stripes.
- What it is
- The deepest-changed of the common metamorphic rocks — a step short of melting. Its minerals have separated into alternating light bands of quartz and feldspar and dark bands of mica and amphibole.
- How to know it
- Striped, and the stripes are usually contorted rather than straight. Coarse enough to see individual grains. Harder and less flaky than schist, and unlike granite the minerals are segregated rather than evenly speckled.
- How it wears
- Very slowly, and by breaking rather than dissolving. Glaciers scraped it bare and it has barely healed since.
- What it means above ground
- Thin, sour, stony soil over rounded bare knobs, with lakes wherever a hollow holds water. Nothing in it neutralises acid, so those lakes are soft, dark, clear and easily acidified by what falls in the rain.
- Where you stand on it
- The Canadian Shield across Quebec, Ontario and Manitoba, the Adirondacks, the Blue Ridge, and the cores of the western ranges.
- What lives on it
- Spruce, pine and birch; blueberry, teaberry and Labrador tea; reindeer lichen on the bare rock. Lake trout and smallmouth bass in the soft water.
- What it's used for
- Dimension stone, aggregate, and — because it is stable and impermeable — the rock chosen to host deep waste repositories.
- What it does to the soil
- Sours the soil above it — nothing in it neutralises rain, so the ground stays acid.
Next to it in the cycle
Paragneiss
gneiss with a sedimentary past
Granite
what gneiss often started as, and what it nearly becomes
Schist
the same journey, stopped earlier
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General descriptions of rock families. Any particular outcrop can differ — rocks grade into one another, and a geological map at this scale describes a district rather than a hillside.